1. Technical Field of the Invention
The present invention relates to exhaust gas cleaning apparatuses (or exhaust emission control devices) for internal combustion engines.
2. Description of the Related Art
A Lean NOx Trap (LNT) is generally disposed in the exhaust pipe of a lean burn internal combustion engine, such as a diesel engine, to reduce the nitrogen oxides (NOx) contained in the exhaust gas from the engine. The LNT is configured to absorb the NOx when the exhaust gas is lean and reduce the absorbed NOx to harmless nitrogen (N2) and water (H2O) when the exhaust gas becomes rich.
Moreover, there are known three approaches for making the normally lean exhaust gas turn to rich. The first approach is to perform rich-burn combustion, that is, to increase the quantities of fuel injected into cylinders of the engine, thereby making the air/fuel mixture in the cylinders rich. The second approach is to perform a post injection in each of the cylinders when the combustion in the cylinder caused by a main injection has almost completed. The third approach is to install an additional fuel injector on the exhaust pipe to inject fuel directly to the LNT.
In addition, there are also known various approaches for improving the performance of a LNT. For example, Japanese Patent First Publication No. 2006-336518 discloses an exhaust emission control system which estimates the deterioration degree of a LNT based on a detection value of an oxygen concentration sensor and sets the length of a lean-burn combustion period based on the estimated deterioration degree.
Among the above-mentioned three approaches for making the exhaust gas turn from lean to rich, the first approach is most advantageous in terms of NOx reduction efficiency and minimization of additional fuel injection.
However, in applying the first approach, there is a problem of “torque shock” to be solved. The problem will be described hereinafter with reference to FIG. 5.
FIG. 5 shows changes in the opening degree of an intake throttle, the opening degree of an EGR (Exhaust Gas Recirculation) valve, command injection quantity, excess oxygen ratio, the air/fuel ratio, and the torque of the engine during a time period of shifting the combustion in the cylinders of the engine from a lean-burn mode to a rich-burn mode.
More specifically, at a timing to, an ECU (Electric Control Unit) for controlling operation of the engine issues a command to shift the combustion in the cylinders of the engine from the lean-burn mode to the rich-burn mode. Then, the opening degree of the intake throttle is decreased in one step, while the opening degree of the EGR valve is increased in one step. As a result, the quantity of air inducted into each of the cylinders of the engine is gradually decreased, thereby decreasing both the excess oxygen ratio and air/fuel ratio in each of the cylinders.
Hereinafter, the excess oxygen ratio denotes the ratio of the quantity of oxygen currently present in each of the cylinders of the engine to the quantity of oxygen present in each of the cylinders when the air/fuel ratio is stoichiometric. It should be noted that the excess oxygen ratio is equal to the excess air ratio in each of the cylinders provided that the percentage of oxygen in air is constant. In addition, the opening degrees of the intake throttle and EGR valve in each of the lean-burn and rich-burn modes may be preset to suitable values.
Further, at a timing t1 later than the timing t0, the command injection quantity is increased in one step. Then, the quantity of fuel injected into each of the cylinders of the engine is accordingly increased. As a result, both the excess oxygen ratio and air/fuel ratio in each of the cylinders are her decreased, thereby making the exhaust gas rich.
The purpose of setting the timing t1 later than the timing t0 is to increase the quantity of fuel injected into each of the cylinders of the engine after decreasing the quantity of air in each of the cylinders, so as to suppress the increase in the torque of the engine which is caused by the increase in the quantity of fuel. In addition, the time interval between the timings t0 and t1 may be preset in consideration of the decrease in the quantity of air in each of the cylinders which is caused by the decreases in the opening degrees of the intake throttle and EGR value.
However, in some cases, the quantity of air in each of the cylinders of the engine cannot be sufficiently decreased by the timing t1 due to changes in the ambient condition and the manufacturing tolerances of the intake throttle and EGR valve. As a result, as shown in FIG. 5, a torque shock (i.e., a rapid increase in the torque of the engine) is ca-used by the increase in the command injection quantity at the timing t1.
The above-described problem of torque shock has not yet been successfully solved by the prior art including Japanese Patent First Publication No. 2006-336518.